1. Field of the Invention
The present invention relates generally to ink-jet hard copy apparatus and methods of operation and, more specifically to a low flow vacuum platen with minimal airflow induced drop directionality errors.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in OUTPUT HARDCOPY [sic] DEVICES, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). As providing background information, the foregoing documents are incorporated herein by reference. Further details of basic ink-jet printing technology are also set forth below in the Detailed Description of the present invention with respect to FIG. 1.
It is known to use a vacuum induced force to adhere a sheet of flexible material to a surface, for example, for holding a sheet of print media temporarily to a transport system or platen. [Hereinafter, xe2x80x9cvacuum induced forcexe2x80x9d is also referred to as xe2x80x9cvacuum induced flow,xe2x80x9d xe2x80x9cvacuum flow,xe2x80x9d or more simply as just xe2x80x9cairflow,xe2x80x9d xe2x80x9cvacuumxe2x80x9d or xe2x80x9csuction,xe2x80x9d as best fits the context.] Such vacuum holddown systems are a relatively common, economical technology to implement commercially and can improve hard copy apparatus throughput specifications. For example, it is known to provide a rotating drum with holes through the surface wherein a vacuum type airflow through the chamber formed by the drum cylinder provides a suction force at the holes in the drum surface (see e.g., U.S. Pat. No. 4,237,466 for a PAPER TRANSPORT SYSTEM FOR AN INK JET PRINTER (Scranton)). [The term xe2x80x9cdrumxe2x80x9d as used hereinafter is intended to be synonymous with any curvilinear implementation incorporating the present invention; while the term xe2x80x9cplatenxe2x80x9d can be defined as a flat holding surface, in hard copy technology it is also used for curvilinear surfaces, e.g., as the ubiquitous typewriter rubber roller; thus, for the purposes of the present application, xe2x80x9cplatenxe2x80x9d is used generically for any shape paper holddown surfacexe2x80x94stationary or movablexe2x80x94as used in a hard copy apparatus.] Permeable belts traversing a vacuum inducing support have been similarly employed (see e.g., Scranton and U.S. Pat. Appl. Ser. No. 09/163,098. by Rasmussen et al. for a BELT DRIVEN MEDIA HANDLING SYSTEM WITH FEEDBACK CONTROL FOR IMPROVING MEDIA ADVANCE ACCURACY (assigned to the common assignee of the present invention and incorporated herein by reference)).
Generally in a hard copy apparatus implementation, the vacuum device is used either to support cut-sheet print media during transport to and from a printing station of a hard copy apparatus, to hold the sheet media at the printing station while images or alphanumeric text are formed (also known as the xe2x80x9cprint zonexe2x80x9d or xe2x80x9cprinting zonexe2x80x9d), or both. [In order to further simplify description of the technology and invention, the term xe2x80x9cpaperxe2x80x9d is used hereinafter to refer to all types of print media and the term xe2x80x9cprinterxe2x80x9d to refer to all types of hard copy apparatus; no limitation on the scope of the invention is intended nor should any be implied.]
In essence, the ink-jet printing process involves digitized, dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent paper. One or more ink-jet type writing instruments (also referred to in the art as an xe2x80x9cink-jet penxe2x80x9d or xe2x80x9cprint cartridgexe2x80x9d) include a printhead which generally consists of drop generator mechanisms and a number of columns of ink drop firing nozzles. Each column or selected subset of nozzles (referred to in the art as a xe2x80x9cprimitivexe2x80x9d) selectively fires ink droplets (typically each being only a few picoliters in liquid volume) that are used to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or xe2x80x9cpixel.xe2x80x9d). Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention.
As the ink-jet writing instrumentsxe2x80x94often scanning at a relatively high rate, across the paperxe2x80x94expel minute droplets of ink onto adjacently positioned print media and sophisticated, computerized, dot matrix manipulation is used to render text and form graphic images, the flight trajectory of each drop is critical to print quality. Printing errors (also referred to in the art as xe2x80x9cartifactsxe2x80x9d) are induced or exacerbated by any airflow in the printing zone. Thus, use of a vacuum platen and vacuum transport device in the printing zone of an ink-jet printer creates an added difficulty for the system designer.
There is a need for a vacuum system for use in an ink-jet printing zone which will provide a minimal airflow impact on ink-jet drop flight trajectory.
In its basic aspects, the present invention provides a print media vacuum platen system including: a vacuum box, having at least one surface thereof further comprising filter mechanisms for permitting airflow therethrough; associated with the vacuum box, vacuum mechanisms for creating a negative pressure within the vacuum box and inducing the airflow through the filter mechanisms; and mounted adjacently to the filter mechanisms distally from the vacuum box, platen mechanisms for holding print media in a position for ink-jet printing thereon, the platen mechanisms having a plurality of vacuum passages therethrough such that regions of the filter mechanisms form a porous floor for each of the passages.
In another basic aspect, the present invention provides a method for providing a substantially uniform airflow across an ink-jet print media vacuum platen associated with a vacuum inducing mechanism. The method includes the steps of: drawing a vacuum through a plurality of vacuum ports distributed across the platen; and filtering the airflow through the ports via an airflow restrictive porous material filter interposed between the platen and the vacuum inducing mechanism.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus including: an ink-jet writing instrument associated with a printing zone within the apparatus; an endless loop vacuum belt system for transporting print media to and from the printing zone; a vacuum platen system located proximate the printing zone, the vacuum platen system having a platen, having a plurality of vacuum ports therethrough, a vacuum chamber, having one wall thereof fabricated of a porous material, the one wall being adjacent the platen such that the material forms a flooring for each of the ports, a vacuum device for maintaining a negative pressure within the chamber such that an airflow is established through the vacuum ports into the chamber via the porous material such that a substantially uniform vacuum force is exerted across the media regardless of the number of vacuum ports covered or partially covered by the print media.
Some of the advantages of the present invention are:
it provides a low flow vacuum system with minimal airflow induced ink drop directionality errors;
it provides a substantially uniform vacuum field regardless of degree of platen coverage;
it provides a low flow platen that allows vacuum box pressure to remain relatively constant whether or not paper is fully covering the platen, thus compensating for different sized print media;
it allows for various media sizes and thicknesses to be held down with substantially the same pressure without requiring a large vacuum source;
it reduces acoustic levels caused by a vacuum induced airflow;
it provides a platen that is resistant to clogging by ink and paper dust; and
it provides improved vacuum holding at paper edges.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.